1. Field of Invention
The present invention relates to a surface light source device used for backlighting an object-to-be-illuminated such as LCD panel or advertising panel, or used for to interior illumination devices, further relating to a light guide plate employed in the surface light source device and a display such as LCD provided with the surface light source device.
2. Related Art
In general, a light guide plate employed in a surface light source device has two major faces and a side peripheral face bridging them. Typically, the major faces are rectangular and the side peripheral face consists of tow pairs of side faces (four side faces). Such a light guide plate is called “rectangular light guide plate” in the instant specification.
It is known well to supply light to a light guide plate through an incidence face provided by a part of a side peripheral face. If a rectangular light guide plate is used, an incidence face is allowed to be formed at a corner portion. This way is suitable particularly for cases where any small type light source (such as LED) is employed as primary light source (light source for supplying light to a light guide plate).
Referring to FIG. 17 showing an example (first prior art) of such a way, surface light source device 101 has a rectangular light guide plate 104 and LED (point-like light source) 106. A corner portion of light guide plate 104 is cut off obliquely to provide incidence face 105.
LED (point-like light source) 106 is disposed opposite to incidence face 105, light from which enters into light guide plate 104 through incidence face 105. The light taken into light guide plate 104 propagates therein. It is noted that “propagating within a light guide plate” is called simply “inner propagation” and “light propagating within a light guide plate” is called simply “inner propagation light” in the instant specification.
On the way of inner propagation, light component having an incident angle not greater than critical angle to emission face 107 of light guide plate 104 is emitted from emission face 104 and transmits through light control members 114 such as light diffusion sheet disposed opposite to emission face 107, then illuminating an object-to-be-illuminated such as LCD panel 103.
In general, LED 106 emits light has a certain diverging angle which is not so large usually. This causes accordingly an inner propagation light to have a not so large diverging angle. As a result, some parts fails to receive sufficient inner propagation light. In particular, enough light hardly reach areas close to which incidence face 105 is connected to side faces 115, 116, respectively, with the result that dark portions tend to appear in the areas.
FIGS. 18a to 18d illustrate a second prior art employable to overcome such problem, which is disclosed in Document 1 noted below. FIG. 18a is a plan view of a light guide plate employed in the second prior art and FIGS. 18b and 18c are side views of the light guide plate, as viewed from one side and another side thereof, respectively. FIG. 18d is an enlarged partial view of part D in FIG. 18a. 
Referring to FIGS. 18a to 18d, light guide plate 104 has incidence face 105 provided with many V-like grooves (recesses) 117 or prismatic projections. V-like grooves (recesses) 117 look “isosceles-like” cut-off portions, as viewed from the above emission face 107. Light from LED 106 is diverged by recesses 117 or prismatic projections, becoming an inner propagation light having diverged travelling directions. As a result, light reaches areas near to sides 115, 116 sufficiently.
This enables an “effective light emitting area” to be provided. It is noted that “effective light emitting area” is an effectively used part of emission face 107 of light guide plate 104 for actually taking out illumination light, such as area 126 surrounded by a rectangular dotted-line shown in FIG. 18a. LCD panel 103 is backlighted by illumination light outputted from effective light emitting area 106.
This art prevents areas close to both sides 115 and 116 from short of brightness. However, there arises another problem, namely, brightness unbalance in an “effective light emitting area”.
Referring to FIG. 18a, reference numerals 115a and 116a denote long and short sides of effective light emitting area 126, respectively. Further, an “incidence reference face” is formed by cutting off corner portion C1 along an imaginary plane that is perpendicular to an imaginary line angle-bisecting an angle made by sides 115a and 116a. 
It is noted that “incidence reference face” corresponds to an imaginary plane which is obtained by imaginarily removing recesses 117 or prismatic projections from incidence face 105.
On the other hand, LED 106 is orientated so that “primary optical axis” is perpendicular to “incidence reference face”. It is noted that “primary optical axis” (i.e. optical axis of primary light) is defined as a travelling direction of light at a center of three-dimensional emission flux (primary light) from LED 106, in the instant specification. In addition, “inner propagation optical axis” (i.e. optical axis of inner propagation light) is defined as a travelling direction of light at a center of three-dimensional inner propagation light, in the instant specification.
In FIG. 18a, the imaginary line (perpendicular to the incidence reference face) angle-bisecting an angle made by sides 115a and 116a accords with inner propagation optical axis 120. Emission face 107 of light guide plate 104 is divided into first emission part 107a and second emission part 107b by inner propagation optical axis 120. Attention should be paid to a fact that diagonal 121 extending from a corner of effective light emitting area 106 corresponding to corner portion C1 does not accord with inner propagation optical axis 120 and passes first emission part 107a. 
Taking into account that light energy distributes on both side of inner propagation optical axis 120 generally symmetrically, first emission part 107a tends to have a reduced brightness as compared with second emission part 107b. That is, an inner propagation light distributing symmetrically with respect to propagation optical axis 120 gives first emission part 107a an emission amount per unit area smaller than that which is given to emission part 107b. This results in an unbalance in brightness between first and second emission parts 107a and 107b. 
FIGS. 19a, 19b illustrate an prior art (third prior art) to overcome such a problem.
FIG. 19a is a plan view of a light guide plate employed in the third prior art and FIG. 19b is a side view of the light guide plate shown in FIG. 19a, as viewed from one side thereof. In addition, FIG. 19c is a side view of the light guide plate shown in FIG. 19a, as viewed from another side thereof. It is noted that the third prior art is disclosed in Document 2 noted below.
According to the third prior art, an incidence face configuration as employed in surface light source device 101 shown in FIGS. 19a and 19b is applied to the surface light source device shown in FIGS. 18a to 18d. In other words, if an incidence face configuration such that inner propagation optical axis 120 is inclined to a long side of effective light emitting area 126 as illustrated in FIGS. 19a and 19b is employed, it is guessed that first and second emission parts 107a and 107b are supplied with generally the same light quantity.
However, this art can brings a dark portion (hatched portion) 119 in the vicinity of one side 106 in a case where light guide plate 104 is shaped like a slant rectangle (in particular, one having a large length-breadth-ratio).                Document 1; JP-A-2003-331628 (Tokkai-2003-331628)        Document 2; JP-A-1999-133425 (Tokkai-Hei 11-133425)        
FIGS. 20a to 20e illustrate an prior art (forth prior art) to overcome such a shortage.
FIG. 20a is a plan view of a light guide plate employed in a forth prior art and FIGS. 20b, 20c are side views of the light guide plate shown in FIG. 20a, as viewed from one and another sides thereof, respectively. Further, FIG. 20d is an enlarged partial view of part E in FIG. 20a and FIG. 20e is an enlarged partial view of part F in FIG. 20d, and FIG. 20f is a plan view of a light guide plate for showing how unevenness in brightness occurs.
According to the forth prior art, incidence face 105 is provided with triangle-like recesses 125, each having a pair of slopes 123 and 124 which have inclination angles with respect to a incidence reference face different from each other, thereby aiming to provide a uniform brightness.
However, this art involves a problem. Referring to FIG. 22e, two light beams 122a, 122b emitted from LED 106 are parallel to each other, both being perpendicular to the incidence reference face.
Beam 122a represents light incident to slope 124 having a small inclination angle and beam 122b represents light incident to slope 123 having a large inclination angle. After incidence, beam 122a becomes inner propagation light beam 122a′ and beam 122b becomes inner propagation light beam 122b′. 
Refraction angle of beam 122b′ at incidence is larger than that of beam 122a′ (θ1<θ2) because inclination angle of slope 123 is larger than that of slope 124. As a result, light supply to vicinage of 116 is increased.
However, this involves a tendency that effective light emitting area 1026 has dark area 127 appearing in the vicinity of side 115 as shown in FIG. 20f, since inner propagation optical axis 120 is urged to be deflected toward side 116 as compared with cases where symmetric slopes are formed. In addition, effective light emitting area 126 tends to has excessively bright area 128 appearing in the vicinity of side 116.
As described above, the above prior arts employ various ideas aiming to uniformalize emission brightness of light guide plate 104, resulting in being not sufficient for uniformalizing emission brightness.